Fuel cells are well known and are commonly used to produce electrical current from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power various types of electrical apparatus. Many fuel cells use a liquid electrolyte such as phosphoric acid, and such fuel cells are typically stacked in series to form a well-known fuel cell stack assembly having manifolds and associated plumbing to deliver and remove reactant and exhaust streams, etc.
In the PAFC, a fixed amount of acid is added in the cell, which is referred to as “beginning of life” or “BOL” acid weight. After addition, acid redistributes within the fuel cell components due to the different pore-sizes of components. Electrolyte retaining matrix pores are always 100% filled. This is required for fuel cell operation, otherwise anode and cathode gas would cross through the matrix and become mixed. After filling the matrix layer, remaining acid is distributes into pores of anode and cathode substrate layer before the PAFC commences operation. Typical BOL fill percent in anode and cathode is thirty to forty percent. Some PAFC locations may see a little higher fill percent in the plane form.
During the operational life of the PAFC, acid is lost from the substrate layers due to evaporation into the reactant and product gases. Such loss will decrease the percent fill in the substrate layer during operation. When the average fill level in a substrate reaches one to two percent, then the matrix starts losing the acid. When this condition is reached it is considered that the fuel cell stack has reached its end of life or “EOL”. Acid loss is dependent on many factors including a rate of evaporation of the acid into the reactant streams. Therefore, it is goal to have as much acid as possible in the fuel cell stack, or in other words to maximize the BOL acid weight.
Acid can transfer within components of a PAFC by electrochemical pumping or “slosh” during fuel cell operation. This intra-cell acid transfer between the cathode and anode results in different electrolyte content in the porous component on each side of the cell. Acid moves from cathode to anode and increased the anode fill level. A cell which begins with the acid equally divided between the anode and cathode components typically ends up with two-third of the acid on the anode and one-third on the cathode at BOL as a result of normal PAFC operation. Additionally, the BOL fill level on the anode side fuel cell components must not exceed a fill level which is required to obtain sufficient gas phase diffusion through the anode side fuel cell components. Therefore, a fill level of the anode side components is very critical when establishing the initial electrolyte fill level for the cell.
Many efforts have been undertaken to increase a “beginning of life” (“BOL”) quantity or weight of acid within a phosphoric acid fuel cell (“PAFC”). For example, U.S. Pat. Nos. 4,035,551 and 4,185,145 both disclose electrolyte reservoirs and methods of making reservoir layers in such fuel cells. These patents include use of varying pore sizes and varying hydrophilic properties to enhance storage of liquid acid electrolytes. U.S. Pat. Nos. 4,345,008 and 4,414,291 also teach PAFCs that include condensation zones to precipitate volatilized acid electrolyte out of a reactant gas stream before the stream leaves the cell to limit acid loss. More recently, U.S. Pat. No. 7,678,478 discloses use of a particular electrolyte retaining matrix having a low phosphoric acid loss rate wherein the cell also includes a condensation zone to further increase retention of the phosphoric acid electrolyte within the fuel cell. (The aforesaid five patents are owned by the owner of all rights in the present disclosure.) Other disclosures such as U.S. Pat. No. 4,467,019 that issued on Aug. 21, 1984 shows use of an external reservoir containing acid electrolyte that may be directed into a PAFC to replace lost electrolyte.
While such disclosures have resulted in a longer operating life of PAFCs, they nonetheless include complex and costly structures and manufacturing methods that limit their effectiveness. Therefore, there is a need for an improved, efficient PAFC that increases a BOL weight of acid and that does not diminish fuel cell performance.